Hydrothermal mineral deposits are accumulations of valuable minerals which formed from hot waters circulating in Earth's crust through fractures. They eventually create metallic-rich fluids concentrated in a selected volume of rock, which become supersaturated and then precipitate ore minerals. In some occurrences, minerals can be extracted for a profit by mining. Discovery of mineral deposits consumes considerable time and resources and only about one in every one thousand prospects explored by companies are eventually developed into a mine. A mineral deposit is any geologically significant concentration of an economically useful rock or mineral present in a specified area. The presence of a known but unexploited mineral deposit implies a lack of evidence for profitable extraction.
Hydrothermal mineral deposits are divided into six main subcategories: porphyry, skarn, volcanogenic massive sulfide (VMS), sedimentary exhalative (SEDEX), and epithermal and Mississippi Valley-type (MVT) deposits. Each hydrothermal mineral deposit has different distinct structures, ages, sizes, grades, geological formation, characteristics and, most importantly, value. Their names derive from their formation, geographical location or distinctive features.
Generally, porphyry-type mineral deposits form in hydrothermal fluid circulation systems developed around felsic to intermediate magma chambers and/or cooling plutons. However, they did not precipitate directly from the magma. While, a skarn deposit is an assemblage of ore and calc-silicate minerals, formed by metasomatic replacement of carbonate rocks in the contact aureole of a pluton. Volcanogenic massive sulfide deposits form when mafic magma at depth, (perhaps a few kilometers beneath the surface), acts as a heat source, causing convective circulation of seawater through the oceanic crust. The hydrothermal fluid leaches metals as it descends and precipitates minerals as it rises. Sedimentary exhalative deposits, also called sedex deposits, are lead-zinc sulfide deposits formed in intracratonic sedimentary basins by the submarine venting of hydrothermal fluids. These deposits are typically hosted in shale. Hydrothermal epithermal deposits consist of geological veins or groups of closely spaced geological veins. Finally, Mississippi Valley-type (MVT) are hosted in limestone or dolomite that was deposited in a shallow marine environment in a tectonically stable intraplate environment. As expected in such an environment, volcanic rocks, folding and regional metamorphism are absent as a general rule. MVT deposits commonly lie in close proximity to evaporites.
Background
A mineral ore deposit is the volume of rock that can be mined at a profit. Therefore, there are many variants that can define whether a mineral deposit is profitable or not, such as price, tonnage, or location. Mineral commodities can be classified as metals or non-metals. Metals refer to elements of the periodic table which include base, ferrous, minor fissionable and precious metals. On the other hand, non-metals refer to industrial minerals such as gypsum, diamonds, oil, coal and aggregate. Hydrothermal deposits of economically valuable and recoverable minerals are generally considered scarce, meaning such deposits are very small relative to the total area of earth's surface.
Each of these deposit types is usually considered to represent a distinctive deposit group with common features and in a similar four-dimensional geodynamic context. The formation of deposits of a particular type can vary in time and location, but different deposit types may also form synchronously, but spatially separated within the same broad orogen.
Hydrothermal mineral deposits play a key role in nearly all modern industrial activities.
According to some authors, the hydrothermal solutions can have four origins, although any single volume of hydrothermal solution is commonly a mixture of two or more types:
Deuteric fluid derived from magma at a late stage of crystallization
Metamorphic fluid derived by progressive removal of hydrothermal fluids during regional metamorphism Meteoric water descending from the surface Fluid formed by the degassing of the core? and mantle Basinal brines are also considered as a possible source of hydrothermal fluids. The fluids are thought to be connate water expelled from sediments with compactions and tectonic forces.